A number of superalloys are gamma prime strengthened nickel based superalloys and are used extensively for high temperature turbine components such as vanes and ring segments. One such superalloy is Inconel 939 (IN939), which is known to have a composition, in weight %, of about 22.0-22.8% Cr, about 18.5-19.5% Co, about 3.6-3.8% Ti, about 1.8-2.0% Al, about 1.8-2.2% W, about 0.9-1.1% Nb, about 1.3-1.5% Ta, about 0.13-0.17% C, and the balance comprising essentially Ni. After a superalloy component casting is formed or developed, it may be subjected to several heat treatments, such as a solution anneal heat treatment, stabilizing heat treatment and aging heat treatment, to strengthen the alloy and component by precipitation of the gamma prime phase in the gamma phase matrix. Although the strengthening gamma prime phase imparts desirable high temperature mechanical properties such as good tensile strength and creep resistance, it also reduces the weldability.
New components such as turbine vanes and ring segments are produced using an investment casting process; but, it is frequently necessary to weld these components both during post-cast manufacturing operations and during repair. However, some nickel based super alloys, such as the IN939 alloy, are difficult to weld without causing cracking when in the standard solution and aged condition. That is, the welding process may place strains at weld locations, which may cause cracking during welding or during the above-referenced post casting heat treatments.
Accordingly, the superalloy castings are often subjected to pre-weld heat treatment processes in order to alleviate potential cracking that may occur during welding or during heat treatments necessary to cause precipitation of the gamma prime phase and strengthen the superalloy. Such pre-weld heat treatments result in “overaging” (growing) the gamma prime phase to produce a coarse gamma prime structure. While these treatments may reduce mechanical properties of the casting or component, the treatments also reduce the propensity of the alloy to exhibit strain age cracking during welding and post weld heat treatments. Although prior art pre-weld heat treatments may effectively achieve a desired ductility of the superalloy to avoid strain age cracking, these procedures can be extremely time consuming due to the ramped heating and cooling stages and holding stages. Thus, the pre-weld heat treatments generally increase the complexity and costs of the manufacturing process of turbine components.